This invention relates to digital wireless communications systems and, more particularly, to transmission channels subject to disturbances such as frequency-selective fading and multipath effects.
In high data throughput wireless digital communication systems, the maximum data transmission rate may be limited by disturbances in the wireless propagation path (i.e., the communication channel). These effects include disturbances such as frequency-selective fading and multipath (copies of the transmitted signal delayed in time to the receiver due to reflections from objects and atmospheric phenomena between the transmitter and the receiver). These disturbances may result in interference between the digitally-modulated symbols representing the information bits to be transmitted, thus impairing the receiver's demodulator from correctly decoding the received symbols to arrive at accurate bit decisions. This “intersymbol interference” may cause the received symbols to overlap the decision boundaries in the complex signal space to adjacent symbols and result in either bit decision errors or lowered bit decision confidence. Systems subject to these effects may thus be required to operate at lower data throughput rates or higher error rates than would otherwise be attainable.
A traditional solution to such channel disturbance problems is to provide an adaptive equalizer consisting of a digital filter whose coefficients can be adjusted to model the inverse of the actual channel impulse response. The resulting digital filter thus enables compensation for the effects of channel nonlinearity by providing this reciprocal of the actual channel impulse response (e.g., a polarity-inverted representation of the channel transmission characteristics as degraded by whatever such disturbances are actually present at a particular time). The determination of the channel impulse response is typically performed by transmitting a test pattern (i.e., a training sequence) to excite the channel at all frequencies, or all frequencies of significant interest, within the data bandwidth of interest and measuring the resulting effect on the training sequence waveform upon transmission through the channel. The calculation of filter coefficients to model the channel impulse response based upon this measurement may typically be done using an estimation process such as a Mean Square Error algorithm.
In digital transmission systems operating at high data rates (such as military communications systems or for commercial wireless Internet access), the channel impulse response estimation time (i.e., the time required to provide such filter coefficients) becomes a critical factor. Since the disturbance effects may be constantly changing, the impulse response estimate must be updated frequently to accommodate high data rates. As a result, the time required to calculate the channel impulse response may become an important factor limiting the maximum data rate of the system. Established techniques and methods for calculating the channel impulse response have typically been subject to constraints on speed, accuracy or other relevant factors.
Objects of the present invention are, therefore, to provide channel impulse response estimation methods and systems which are new or improved and which may provide one or more of the following capabilities or characteristics:                improved capability to estimate channel impulse response;        rapid estimation of channel impulse response;        channel impulse response estimation repetition on per packet basis;        provision of an improved form of training sequence;        provision of a training sequence enabling formulation of a Toeplitz-type mathematical system or matrix at a receiver;        construction of a Toeplitz-type matrix representation usable to determine coefficients representative of a channel impulse response; and        use of fast algorithms, such as Levinson algorithms, to determine coefficients representative of a channel impulse response.        